1. Field of the Invention
The present invention relates to an image forming device for forming an image on a photo sensitive paper by scanning laser beams of three colors, red, green and blue, on the photo sensitive paper by a one dimensional scanning device such as polygon mirror, and by feeding the photo sensitive paper in a direction transverse to a scanning direction, and further by modulating a strength of each color laser beam by synchronizing the beam with the scanning device More particularly, the present invention relates to a mechanism for synchronizing the scanning device and a timing of sending data representative of each color of an image data for an amplitude modulation so as to compensate for chromatic divergence or distortion (chromatism).
2. Description of Related Art
Conventionally, in this kind of image forming device, a laser beam of three colors was scanned by a polygon mirror after these three beams became one beam, as described, for example, in A Continuous-Tone Laser Color Printer by Firth et al. (Journal of Imaging Technology, Volume 14, Number 3, Jun. 1988). In this conventional image forming device, a photosensor which reacts to a light having a specific wavelength was set to detect a part of the beam that was separated by a beam splitter from the scanning beam path and thus did not contribute to the image formation on the scanning lines. The sending of the timing of an image data was controlled based on a detection signal from the sensor. Moreover, the distance between the inclination center of the reflecting mirror and the recording medium is enough to make both an image width and the long focus satisfactory so that the inclination scanning angle becomes narrow and distortion of the image is suppressed.
However, in order to suppress the distortion of the formed image, the distance between the inclination center of the reflecting mirror and the recording medium results in the device becoming large. Further, because the inclination scanning angle is narrow, the scanning efficiency is low in the 6 and 8 sided polygon mirrors conventionally used and the image formation speed of the device becomes slow. In the case where the number of sides of the polygon mirror is increased, the manufacturing cost of the mirror increases and the device becomes expensive. Further, the light beam diameter on the inclination scanning center becomes large, and the diameter of the polygon mirror becomes large, so that a large-scale motor is needed to rotate the polygon mirror. Moreover, a strong bearing for supporting the rotation axis is needed and the cost of the device is further increased.
Moreover, even if a f.theta. lens, in which an image width is in proportion to an angle of incidence, is used in order to avoid the above-mentioned defect, it is difficult to manufacture a scanning lens having enough condensing performance for focusing and keeping the same image width in the scanning direction for the light of the three different wavelengths due to chromatic distortions (chromatism).